This invention relates to an electronic power supply unit known as the switching-mode power supply, and more specifically, to such a power supply of the kind having a synchronous rectifier circuit.
A typical conventional switching-mode power supply with a flyback DC-to-DC converter includes a transformer having a primary winding coupled to the pair of DC input terminals via an active switch, and a secondary winding coupled to the pair of DC output terminals via a rectifying diode and smoothing capacitor. The DC input voltage is rapidly turned on and off as the active switch is driven by pulses that have been width-modulated by a feedback circuit monitoring the DC output voltage. Energy is stored on the transformer during the conducting periods of the active switch and released during its nonconducting periods. The smoothing capacitor is charged as the rectifying diode conducts during the nonconducting periods of the active switch.
A voltage drop of approximately 0.8 volt has been known to occur across the rectifying diode of the above switching-mode power supply. Japanese Unexamined Patent Publication No. 9-163736 teaches how to reduce this voltage drop, and consequent power loss, across the rectifying diode. Connected in parallel with the rectifying diode according to this prior art is a synchronous rectifier switch which is turned on during the conducting periods of the rectifying diode. The synchronous rectifier switch, particularly when in the form of a bipolar or field-effect transistor, introduces a voltage drop of as low as 0.2 volt or so, realizing an appreciable diminution of an overall voltage drop on the output side of the transformer.
This prior art technique has proved to possess its own shortcomings, however. One of them is the difficulty of turning on the synchronous rectifier switch in exact synchronism with the conducting periods of the rectifying diode, these conducting periods being subject to change with the input voltage and with the loading.
Another weakness manifested itself in the event of an abrupt drop in the output voltage of the switching-mode power supply in response to a change in loading. Thereupon the standard feedback circuit of the closed-loop switching regulator responded by correspondingly extending the conducting periods of the active switch. The possible result was the undesired overlapping of the conducting periods of the active switch and those of the synchronous rectifier switch. Such overlapping, if it occurred at all, might lead to noise production and, worse yet, to the destruction of the associated circuit parts.
The yet-unremedied drawbacks of the synchronous rectification technology discussed above are not peculiar to the flyback DC-to-DC converter. The same technology has been applied, at the risk of like contingencies, to other varieties of switching-mode power supplies such as a forward DC-to-DC converter and a chopper converter.